DE102013221055A1 - Combination of radar sensor and cowling for a motor vehicle - Google Patents

Abstract

Combination of a radar sensor (10) and a cowling (12) to be mounted on a motor vehicle, that the cowling is irradiated by microwaves (14) of the radar sensor, wherein the cowling has at least one layer (c), which is a part the microwave reflects, characterized in that the covering part (12) has an additional layer (s), which is designed according to the thickness and dielectric constant, to reduce the reflection.

Description

The invention relates to a combination of a radar sensor and a cowling, which are to be mounted on a motor vehicle, that the cowling is irradiated by microwaves of the radar sensor, wherein the cowling has at least one layer which reflects a portion of the microwaves.

Radarsensors are increasingly used in motor vehicles which serve to detect the traffic environment in order to provide data for various driver assistance functions, for example for a distance control or for a lane change assistant.

It is often desirable to conceal the radar sensor invisibly behind a trim part, for example a bumper of the motor vehicle. However, it must be taken into account that some of the microwaves that radiate through the bumper are reflected on this bumper, so that the performance and thus the range of the radar sensor is reduced. Even more serious is that the waves reflected on the bumper and again received by the radar sensor form an interference signal for the evaluation of the actual radar signal. As a result, in particular in the case of angle-resolving radar sensors, the measurement of the angular position of the located radar targets is made more difficult.

In plastic bumpers, which consist homogeneously of a uniform material, it is possible to design the thickness of the plastic wall forming the bumper so that the reflected waves are largely extinguished by resonance and interference effects. For trim parts with a multi-layered structure, such as painted bumpers, however, such a design is not practical.

The object of the invention is therefore to provide a combination of radar sensor and cowling, in which the reflection of the microwaves is reduced, especially in fairing parts with a multi-layered structure.

This object is achieved in that the covering part has an additional layer, which is designed according to the thickness and dielectric constant, to reduce the reflection.

The invention thus makes it possible to design the actual bumper (without the additional layer) solely with regard to the properties usually required by bumpers, such as deformation properties, appearance and the like, without the bumper being used in combination with a radar sensor , the reflection behavior must be taken into account that is also dependent on the wavelength of the microwaves and thus of the operating frequency of the respective radar sensor. In order to optimize the bumper then with a view to use in combination with a radar sensor, the additional layer is applied, whose function is to modify the reflection properties and in particular to achieve that reflected at the reflective layer of the trim piece waves with the The waves reflected destructively interfere with the additional layer, so that an overall reduction of the reflection is achieved.

Advantageous embodiments of the invention are specified in the subclaims.

The additional layer can be formed, for example, by a molded part additionally attached to the bumper or by a film, for example a PVC film. Particularly advantageous is the use of several film layers, which allows to vary the total thickness of the additional layer by a suitable choice of the number of film layers as needed.

In a trim panel for motor vehicles, the most reflective layer is usually formed by a lacquer layer on the outside of the trim panel. The additional layer is then preferably mounted on the opposite side of the trim panel.

In the case of a trim part with a multilayer construction, the optimum combination of thickness and dielectric constant of the additional layer is in principle dependent on the thicknesses and dielectric constants of all other layers of the trim part. In practice, however, often the influence of individual layers of the trim part can be neglected, so that the optimal choice depends on a smaller number of parameters.

The invention also relates to a motor vehicle with the above-described combination of radar sensor and cowling and a method for producing a trim part for motor vehicles.

In the following embodiments are explained in detail with reference to the drawing.

Show it:

1 a schematic view of a radar sensor in combination with a cowling, of which only a wall portion is shown in cross section;

2 a diagram for different thicknesses of an additional layer of the in 1 shown lining part indicates the reflection coefficient as a function of the dielectric constant of a paint layer of this trim part;

3 a fault diagram for angle measurements with the radar sensor after 1 in combination with a conventional bumper without additional layer, indicates the angular error as a function of the locus angle; and

4 a fault diagram for the radar sensor in combination with a bumper according to the invention.

In 1 is schematically a radar sensor 10 shown at some distance behind a cowling 12 , namely a bumper of a motor vehicle is arranged, so that the radar sensor 10 emitted microwaves 14 the bumper 12 have to shine through. In the radar sensor 10 For example, it is a 77 GHz radar, so the wavelength of the microwaves 14 approximately on the order of 4 mm.

The fairing part 12 has a layer structure which is not drawn to scale in the drawing, and comprises a base body a made of plastic with a typical thickness d _a of 2.8 mm, on which of the radar sensor 10 A facing layer b having a thickness of typically about 10 .mu.m, a basecoat layer c having a thickness _d.sub.c of typically about 20 .mu.m and finally a clearcoat layer d having a thickness _d.sub.d of typically about 30 .mu.m are arranged in sequence.

The dielectric constant (relative permittivity) εr _{a of} the plastic material of the base body a is typically between 2 and 4. The undercoat layer _b typically has a relative permittivity εr _b of 8, and the clearcoat layer _d typically has a relative permittivity εr _d of 3.5. The dielectric constant εr _{c of} the basecoat film c can assume values up to 120. The amount of this dielectric constant εr _c is dependent on the desired optical properties and assumes very high values, in particular for coatings with a metallic effect.

For the microwaves 14 In principle, each interface, at which the dielectric constant and thus the optical density of the medium changes, represents a reflection surface on which a part of the incident radiation is reflected. In the case of the layer structure shown here, in particular the basecoat film c acts as a reflective layer because of its high dielectric constant, ie, there are particularly strong reflections at the interfaces that form this layer with the primer layer b and the clearcoat film d. The microwaves that are reflected at the various boundary layers of the fabric scavenger interfere with each other and can structurally or destructively interfere with one another depending on the thickness of the layers and the pairing of the dielectric numbers at the interfaces. These interference effects ultimately determine the reflection coefficient that the trim panel 12 as a whole for the microwaves 14 having.

In 2 shows a dashed line curve f0 the reflection coefficient of a trim part, the only of the layers a, b, c and d in 1 exists as a function of the dielectric constant εr _{c of} the basecoat film c. It can be seen that the reflection coefficient increases sharply with increasing dielectric constant of the basecoat film and can even amount to more than 90% for dielectric numbers above 100.

The reflections are tempered by the fact that the in 1 shown trim part 12 on the radar sensor 10 facing side of the body a has an additional layer e, which forms additional reflection surfaces and thus changes the interference pattern. In the in 1 As shown, the additional layer e consists of two layers of a film 16 , For example, a PVC film with the relative permittivity εr _e = 8 and a thickness of 200 microns. The total thickness d _{e of} the additional layer e is therefore 400 μm.

The reflection coefficient which results in the presence of this additional layer e is in 2 represented as curve f2. It can be seen that this curve has a pronounced minimum approximately at εr _c = 30, so that the reflection at the covering part is almost completely suppressed when the basecoat layer c has a relative permittivity in the vicinity of 30.

By varying the number of layers of the film in the additional layer e (with unchanged thickness and dielectric constant of the individual film 16 ), the reflection behavior for different dielectric constants εr _{c of} the basecoat film can be 10 adjust as indicated by the remaining curves in 2 is illustrated. With only one layer of the film 16 The result is a reflection coefficient, which is indicated by the curve f1. Although this curve does not have such a pronounced minimum, it runs very flat and therefore remains below 60% even at high values of εr _c . This additional layer is therefore a good choice in connection with basecoat films which have very high dielectric constants.

For an additional layer e with three layers of film 16 One obtains a reflection coefficient whose curve f3 is similar to the curve for f2, but has its minimum at about εr _c = 5. In four layers, the reflection coefficient is described by a curve f4, which is also relatively flat, but has no pronounced minimum. The curve f5 indicates the reflection coefficient for five plies and has a minimum between 40 and 50, and the curve f6 indicates the reflection coefficient for six plies and has a minimum between 10 and 20.

In this way, the number of layers of the film can be determined for any given dielectric constant εr _{c of} the basecoat film c 16 so that the additional layer e minimizes the reflection coefficient. However, it should be noted that, due to the periodic nature of the microwaves, there is no simple relationship between the relative permittivity εr _{c of} the basecoat layer and the optimum thickness d _{e of} the additional layer e, so that the optimum thickness of the additional layer is calculated or determined empirically in the individual case got to.

To a certain extent, that will be in 2 is also influenced by the thicknesses and the electricity numbers of the layers a, b and d, so that a different diagram is obtained for each combination of the parameters d _a , d _b , d _d , εr _a , εr _b and εr _d . However, the influence of the layers b and d will in many cases be negligible, so that the number of parameters to be taken into account will be correspondingly reduced.

Of course, the dielectric constant εr _{e of} the film has 16 an effect on the appearance of the chart in 2 , Instead of varying the thickness _e of the additional layer e, it is therefore possible to vary at a constant thickness of the material of this layer and thus the dielectric constant .epsilon..sub.R _e and then select .epsilon..sub.R _e based on the diagram that dielectric constant, which for the given basecoat c gives the lowest reaction coefficient.

Of course, it is also possible to combine in the additional layer e films with different thicknesses and / or with different dielectric constants.

One for use with the radar sensor 10 In the general case, one can proceed as follows:

For a cladding part with n layers (including the additional layer e), one obtains a set of 2n variables, namely the layer thicknesses and the dielectric constants of these layers. All these variables can in principle be varied independently of each other. For a radar sensor with a given operating frequency and accordingly a given wavelength of the microwaves, the reflection coefficient can then be calculated as a function of the 2n independent variables or possibly also determined empirically. The function thus obtained will generally have several local minima. Some of the independent variables will have some limitations. This applies, for example, to the thickness d _{a of} the basic body and, if a coating in a specific color is desired, for the dielectric constant εr _{c of} the basecoat film. Taking these restrictions into account, the deepest local medium can then be found and a lining part with the appropriate layer structure can be produced.

The complexity can be substantially reduced by treating some of the 2n variables as fixed parameters, for example the layer thicknesses and the dielectric constants of the layers a, b and d and optionally also the layer thickness d _{c of} the basecoat layer and the relative permittivity εr _{e of} the additional layer e , so that the task is reduced to finding the minimum of a function in two variables (εr _c and d _e ).

By minimizing the reflection coefficient, in particular in the case of an angle-resolving radar sensor 10 An improvement in the accuracy of the angle measurement can be achieved. This is done by in the 3 and 4 illustrated diagrams illustrated.

In 3 gives a curve 18 the error occurring in the angle measurement of a single object as a function of the angle in the event that the cowling 12 the in 1 layer structure shown without the additional layer e has. It is also in 3 a permissible tolerance range 20 shown. It can be seen that the error often clearly exceeds the permissible tolerances.

In 4 shows a curve 22 the corresponding results in the event that the fairing part 12 has the additional layer e. The angle error is thereby considerably reduced and is largely within the tolerance range 20 ,

Claims (7)

Combination of a radar sensor ( 10 ) and a trim part ( 12 ), which are to be mounted on a motor vehicle, that the covering part of microwaves ( 14 ) of the radar sensor, wherein the covering part has at least one layer (c) which reflects a part of the microwaves, characterized in that the covering part ( 12 ) has an additional layer (e), which according to thickness (d _e ) and dielectric constant (εr _e ) is designed to reduce the reflection. Combination according to Claim 1, in which the trim part ( 12 ) is a bumper. A combination according to claim 1 or 2, wherein the reflective layer (c) and the additional layer (e) are disposed on opposite sides of a main body (a) of the trim panel. Combination according to one of the preceding claims, in which the additional layer (e) is a film ( 16 ) contains. A combination according to claim 4 wherein the additional layer (e) is defined by multiple layers of the film ( 16 ) is formed. Motor vehicle with a combination of radar sensor ( 10 ) and trim part ( 12 ) according to any one of the preceding claims. Method for producing a trim part ( 12 ) for a motor vehicle, for use in combination with a radar sensor ( 10 ), which is arranged on the motor vehicle, that microwaves ( 14 ) of the radar sensor through the covering part, characterized by the following steps: - defining a layer structure for a precursor for the covering part, with a plurality of layers (a, b, c, d), each having a certain thickness (d _a , d _b , d _c , d _d ) and have a certain dielectric constant (εr _a , εr _b , εr _c , εr _d ), - determining a reflection coefficient for the lining part ( 12 ), Which is formed by the primary product and an additional layer (s), as a function of at least one independent variable _(e n) that indicates the thickness of the additional layer (s), - looking up a minimum of this function, and - wring the additional Layer (s) with the minimum thickness (d _e ) on the precursor.

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